U.S. patent number 10,339,967 [Application Number 16/077,632] was granted by the patent office on 2019-07-02 for reproducing apparatus and reproducing method.
This patent grant is currently assigned to SONY CORPORATION. The grantee listed for this patent is SONY CORPORATION. Invention is credited to Toshihiro Horigome, Kotaro Kurokawa, Kimihiro Saito, Koji Sekiguchi.
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United States Patent |
10,339,967 |
Saito , et al. |
July 2, 2019 |
Reproducing apparatus and reproducing method
Abstract
Provided is a reproducing apparatus including: a reproduction
signal generating circuit that calculates a first difference signal
which is a difference between a first light receiving signal
obtained by the first light receiving element and a second light
receiving signal obtained by the second light receiving element,
and a second difference signal which is a difference between a
third light receiving signal obtained by the third light receiving
element and a fourth light receiving signal obtained by the fourth
light receiving element, and uses the first difference signal, the
second difference signal, a phase difference between a crosstalk
component and an average phase of the signal light beam, and an
optical path length difference between the signal light beam and
the reference light beam to obtain a reproduction signal; and a
phase extraction circuit that obtains a successive change amount
and updates with a successive variation.
Inventors: |
Saito; Kimihiro (Saitama,
JP), Horigome; Toshihiro (Kanagawa, JP),
Sekiguchi; Koji (Kanagawa, JP), Kurokawa; Kotaro
(Kanagawa, JP) |
Applicant: |
Name |
City |
State |
Country |
Type |
SONY CORPORATION |
Tokyo |
N/A |
JP |
|
|
Assignee: |
SONY CORPORATION (Tokyo,
JP)
|
Family
ID: |
60160234 |
Appl.
No.: |
16/077,632 |
Filed: |
January 27, 2017 |
PCT
Filed: |
January 27, 2017 |
PCT No.: |
PCT/JP2017/002958 |
371(c)(1),(2),(4) Date: |
August 13, 2018 |
PCT
Pub. No.: |
WO2017/187688 |
PCT
Pub. Date: |
November 02, 2017 |
Prior Publication Data
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|
|
Document
Identifier |
Publication Date |
|
US 20190035425 A1 |
Jan 31, 2019 |
|
Foreign Application Priority Data
|
|
|
|
|
Apr 28, 2016 [JP] |
|
|
2016-090125 |
|
Current U.S.
Class: |
1/1 |
Current CPC
Class: |
G11B
7/0053 (20130101); G11B 7/13 (20130101); G11B
20/10009 (20130101); G11B 7/1395 (20130101); G11B
7/00718 (20130101); G11B 7/005 (20130101); G11B
7/135 (20130101) |
Current International
Class: |
G11B
7/135 (20120101); G11B 7/007 (20060101); G11B
7/005 (20060101); G11B 20/10 (20060101) |
References Cited
[Referenced By]
U.S. Patent Documents
Foreign Patent Documents
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3252772 |
|
Dec 2017 |
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EP |
|
4564948 |
|
Oct 2010 |
|
JP |
|
2013-054801 |
|
Mar 2013 |
|
JP |
|
2014-032728 |
|
Feb 2014 |
|
JP |
|
2015-028823 |
|
Feb 2015 |
|
JP |
|
WO 2013/031120 |
|
Mar 2013 |
|
WO |
|
Other References
Mar. 22, 2019, European Search Report issued for related EP
application No. 17788978.9. cited by applicant.
|
Primary Examiner: Agustin; Peter Vincent
Attorney, Agent or Firm: Paratus Law Group, PLLC
Claims
The invention claimed is:
1. A reproducing apparatus comprising: an optical system that
irradiates a recording medium on which signals are each recorded on
a land and a groove with a light beam emitted from a light source
to obtain a signal light beam reflecting each of the recording
signals of the land and the groove, generates a reference light
beam from the light beam emitted from the light source, and
generates a set of a first signal light beam and a reference light
beam which give a phase difference of approximately 0.degree. to a
superimposed light beam obtained by superimposing the signal light
beam and the reference light beam, a set of a second signal light
beam and a second reference light beam which give a phase
difference of approximately 180.degree. to the superimposed light
beam, a set of a third signal light beam and a third reference
light beam which give a phase difference of approximately
90.degree. to the superimposed light beam, and a set of a fourth
signal light beam and a fourth reference light beam which give a
phase difference of approximately 270.degree. to the superimposed
light beam; a light receiving unit that uses a first light
receiving element to receive the set of the first light beam and
the first reference light beam, a second light receiving element to
receive the set of the second signal light beam and the second
reference light beam, a third light receiving element to receive
the set of the third signal light beam and the reference, and a
fourth light receiving element to receive the set of the fourth
signal light beam and the fourth reference light beam; a
reproduction signal generating circuit that calculates a first
difference signal a which is a difference between a first light
receiving signal obtained by the first light receiving element and
a second light receiving signal obtained by the second light
receiving element, and a second difference signal b which is a
difference between a third light receiving signal obtained by the
third light receiving element and a fourth light receiving signal
obtained by the fourth light receiving element, and uses the first
difference signal a, the second difference signal b, a phase
difference .PSI. between a crosstalk component and an average phase
of the signal light beam, and an optical path length difference
.theta. between the signal light beam and the reference light beam
to carry out an arithmetic operation of
asin(.PSI.-.theta.(t))-bcos(.PSI.-.theta.(t)) to obtain a
reproduction signal; and a phase extraction circuit that obtains a
successive change amount .DELTA..theta. of .theta. and updates
.theta. with the successive variation .DELTA..theta..
2. The reproducing apparatus according to claim 1, wherein a
successive change amount .DELTA..theta.t is obtained by a following
expression when .theta.t+1=.theta.t+.DELTA..theta.t. [Expression
18]
.DELTA..times..times..theta..eta..function..times..times..times..theta..t-
imes..times..times..theta..times..times..times..theta..times..times..times-
..times..times..theta. ##EQU00003##
3. The reproducing apparatus according to claim 1, wherein the
reference light beam is generated by reflecting a light beam
emitted from the light source by a mirror.
4. The reproducing apparatus according to claim 1, wherein the
phase difference is assumed to be approximately equal to
(.PSI.=4.pi.nd/.lamda.) (n is a refractive index, d is a step
between the land and the groove, and .lamda. is a wavelength of a
light beam).
5. The reproducing apparatus according to claim 1, wherein when a
reproduction signal is formed from a light receiving signal output
from a light receiving element, an optical path is divided into a
first optical path and second optical path, a high frequency
component is extracted from a light receiving signal on the first
optical path, a low-frequency component is extracted from a light
receiving signal on the second optical path, and the high frequency
component converted into a digital signal and the low frequency
component converted into a digital signal are synthesized to obtain
a reproduction signal.
6. The reproducing apparatus according to claim 5, further
comprising a reference light beam servo that applies a reference
light beam servo by a light receiving signal on the second optical
path.
7. The reproducing apparatus according to claim 1, wherein when a
reproduction signal is formed from a light receiving signal output
from a light receiving element, an optical path is divided into a
first optical path and second optical path, a high frequency
component is extracted from a light receiving signal on the first
optical path, a reference light beam servo is applied by the light
receiving signal of the second optical path, and the high frequency
component converted into a digital signal and a fixed DC value are
synthesized to obtain a reproduction signal.
8. The reproducing apparatus according to claim 1, wherein when a
reproduction signal is formed from a light receiving signal output
from a light receiving element, an optical path is divided into a
first optical path and second optical path, a fixed DC value is
subtracted from a light receiving signal on the first optical path,
a reference light beam servo is applied by the light receiving
signal of the second optical path, and a signal obtained by
converting into a digital signal a signal in which the fixed DC
value is subtracted and the fixed DC value are synthesized to
obtain a reproduction signal.
9. The reproducing apparatus according to claim 1, wherein when a
reproduction signal is formed from a light receiving signal output
from a light receiving element, a reference light beam servo is
applied by a light receiving signal of a single optical path, a DC
value corresponding to a target phase of the reference light beam
servo is subtracted from the light receiving signal, and a signal
obtained by converting into a digital signal a signal in which the
DC value is subtracted and a signal obtained by converting the DC
value corresponding to the target phase into a digital signal are
synthesized to obtain a reproduction signal.
10. The reproducing apparatus according to claim 1, wherein when a
reproduction signal is formed from a light receiving element output
from a light reception element, a reference light beam servo is
applied by a light receiving signal of a single optical path, a
fixed DC value is subtracted from the light receiving signal, and a
signal obtained by converting into a digital signal a signal in
which the DC value is subtracted and a signal obtained by
converting the fixed DC value into a digital signal are synthesized
to obtain a reproduction signal.
11. The reproducing apparatus according to claim 1, wherein when a
reproduction signal is formed from a light receiving signal output
from a light receiving element, a reference light beam servo is
applied by a light receiving signal of a single optical path, and a
signal obtained by converting the light receiving signal into a
digital signal and a signal obtained by converting a DC value
corresponding to the target phase into a digital signal are
synthesized to obtain a reproduction signal.
12. A reproducing method comprising: irradiating a recording medium
on which signals are each recorded on a land and a groove with a
light beam emitted from a light source to obtain a signal light
beam reflecting each of the recording signals of the land and the
groove, generating a reference light beam from the light beam
emitted from the light source, and generating, by an optical
system, a set of a first signal light beam and a first reference
light beam which give a phase difference of approximately 0.degree.
to a superimposed light beam obtained by superimposing the signal
light beam and the reference light beam, a set of a second signal
light beam and a reference light beam which give a phase difference
of approximately 180.degree. to the superimposed light beam, a set
of a third signal light beam and a third reference light beam which
give a phase difference of approximately 90.degree. to the
superimposed light beam, and a set of a fourth signal light beam
and a fourth reference light beam which give a phase difference of
approximately 270.degree. to the superimposed light beam; using a
first light receiving element to receive the set of the first light
beam and the first reference light beam, a second light receiving
element to receive the set of the second signal light beam and the
second reference light beam, a third light receiving element to
receive the set of the third signal light beam and the third
reference light beam, and a fourth light receiving element to
receive the set of the fourth signal light beam and the fourth
reference light beam; calculating a first difference signal a which
is a difference between a first light receiving signal obtained by
the first light receiving element and a second light receiving
signal obtained by the second light receiving element, and a second
difference signal b which is a difference between a third light
receiving signal obtained by the third light receiving element and
a fourth light receiving signal obtained by the fourth light
receiving element, and using the first difference signal a, the
second difference signal b, a phase difference .PSI. between a
crosstalk component and an average phase of the signal light beam,
and an optical path length difference .theta. between the signal
light beam and the reference light beam to carry out an arithmetic
operation of asin(.PSI.-.theta.(t))-bcos(.PSI.-.theta.(t)) to
obtain a reproduction signal; and obtaining a successive change
amount .DELTA..theta. of .theta. and updates .theta. with the
successive variation .DELTA..theta..
Description
CROSS REFERENCE TO PRIOR APPLICATION
This application is a National Stage Patent Application of PCT
International Patent Application No. PCT/JP2017/002958 (filed on
Jan. 27, 2017) under 35 U.S.C. .sctn. 371, which claims priority to
Japanese Patent Application No. 2016-090125 (filed on Apr. 28,
2016), which are all hereby incorporated by reference in their
entirety.
TECHNICAL FIELD
The present technology relates to a reproducing apparatus and a
reproducing method applied to reproduce an optical medium such as
an optical disk.
BACKGROUND ART
For example, when reproducing a multilayered optical disc, the
signal light intensity decreases, and there is a high possibility
that an error occurs in signal reading. In order to solve this
problem, a homodyne detection method of amplifying a detection
signal by using light interference is known (see Patent Document
1).
In Patent Document 1, as a homodyne system for detecting a light
beam in which a signal light beam and a reference light beam are
made to interfere with each other, it is necessary to perform
detection on sets of four signal light beams and reference light
beams whose phase differences are different by 90 degrees,
respectively. Specifically, detection is performed on the sets of
signal light beams and reference light beams whose phase
differences are set to 0 degrees, 90 degrees, 180 degrees, and 270
degrees, respectively. Each of these detections is performed by
detecting the light intensity of the light beam in which the signal
light beam and the reference light beam are made to interfere with
each other.
Further, Patent Document 2 describes signal processing for
correctly reproducing a signal even when a light receiving element
and an amplifier are AC-coupled. Further, Patent Document 3
describes a reproducing apparatus that applies a homodyne system to
an optical disk in which signals are each recorded on a land and a
groove.
In the homodyne system, the component of the signal light beam
amplified according to the light intensity of the reference light
beam can be obtained as a reproduction signal. By amplifying the
signal light beam in this way, a signal to noise ratio (SNR) of the
reproduction signal can be improved. Furthermore, the obtained
reproduction signal is not influenced by the phase difference
between the signal light beam and the reference light beam, so that
an optical path length difference adjustment (so-called optical
path length servo) can be made unnecessary.
CITATION LIST
Patent Document
Patent Document 1: Japanese Patent No. 4564948
Patent Document 2: Japanese Patent Application Laid-Open No.
2013-54801 A
Patent Document 3: Japanese Patent Application Laid-Open No.
2014-32728 A
SUMMARY OF THE INVENTION
Problems to be Solved by the Invention
Since the amplitude of a DC component is very large with respect to
an AC component, considering the dynamic range of the amplifier,
Patent Document 2 solves the problem that it is impossible to
effectively increase an amplification factor of the AC component
that is desired to be detected truly. In Patent Document 2, the AC
component alone can be amplified by AC-coupling.
In Patent Document 3, the components of sin .theta. and cos .theta.
are extracted from a signal obtained by a low pass filter, and a
phase offset .theta. of the reference light beam is extracted using
these values. However, the method using the low pass filter has a
problem that a desired improvement effect cannot be obtained due to
a reason that a noise enters a band of the reproduction signal
because a noise band is wide.
Accordingly, the purpose of the present technology is to adopt a
homodyne detection method and to provide a reproducing apparatus
and a reproducing method capable of accurately extracting a phase
offset.
Solutions to Problems
The present technology is a reproducing apparatus including:
an optical system that irradiates a recording medium on which
signals are each recorded on a land and a groove with a light beam
emitted from a light source to obtain a signal light beam
reflecting each of the recording signals of the land and the
groove, generates a reference light beam from the light beam
emitted from the light source, and generates a set of a first
signal light beam and a first reference light beam which give a
phase difference of approximately 0.degree. to a superimposed light
beam obtained by superimposing the signal light beam and the
reference light beam, a set of a second signal light beam and a
second reference light beam which give a phase difference of
approximately 180.degree. to the superimposed light beam, a set of
a third signal light beam and a third reference light beam which
give a phase difference of approximately 90.degree. to the
superimposed light beam, and a set of a fourth signal light beam
and a fourth reference light beam which give a phase difference of
approximately 270.degree. to the superimposed light beam;
a light receiving unit that uses a first light receiving element to
receive the set of the first light beam and the first reference
light beam, a second light receiving element to receive the set of
the second signal light beam and the second reference light beam, a
third light receiving element to receive the set of the third
signal light beam and the third reference light beam, and a fourth
light receiving element to receive the set of the fourth signal
light beam and the fourth reference light beam;
a reproduction signal generating circuit that calculates a first
difference signal a which is a difference between a first light
receiving signal obtained by the first light receiving element and
a second light receiving signal obtained by the second light
receiving element, and a second difference signal b which is a
difference between a third light receiving signal obtained by the
third light receiving element and a fourth light receiving signal
obtained by the fourth light receiving element, and uses the first
difference signal a, the second difference signal b, a phase
difference .PSI. between a crosstalk component and an average phase
of the signal light beam, and an optical path length difference
.theta. between the signal light beam and the reference light beam
to carry out an arithmetic operation of
asin(.PSI.-.theta.(t))-bcos(.PSI.-.theta.(t))
to obtain a reproduction signal; and
a phase extraction circuit that obtains a successive change amount
.DELTA..theta. of .theta. and updates .theta. with the successive
variation .DELTA..theta..
The present technology is a reproducing method including:
irradiating a recording medium on which signals are each recorded
on a land and a groove with a light beam emitted from a light
source to obtain a signal light beam reflecting each of the
recording signals of the land and the groove, generating a
reference light beam from the light beam emitted from the light
source, and generating, by an optical system, a set of a first
signal light beam and a first reference light beam which give a
phase difference of approximately 0.degree. to a superimposed light
beam obtained by superimposing the signal light beam and the
reference light beam, a set of a second signal light beam and a
second reference light beam which give a phase difference of
approximately 180.degree. to the superimposed light beam, a set of
a third signal light beam and a third reference light beam which
give a phase difference of approximately 90.degree. to the
superimposed light beam, and a set of a fourth signal light beam
and a fourth reference light beam which give a phase difference of
approximately 270.degree. to the superimposed light beam;
using a first light receiving element to receive the set of the
first light beam and the first reference light beam, a second light
receiving element to receive the set of the second signal light
beam and the second reference light beam, a third light receiving
element to receive the set of the third signal light beam and the
third reference light beam, and a fourth light receiving element to
receive the set of the fourth signal light beam and the fourth
reference light beam;
calculating a first difference signal a which is a difference
between a first light receiving signal obtained by the first light
receiving element and a second light receiving signal obtained by
the second light receiving element, and a second difference signal
b which is a difference between a third light receiving signal
obtained by the third light receiving element and a fourth light
receiving signal obtained by the fourth light receiving element,
and using the first difference signal a, the second difference
signal b, a phase difference .PSI. between a crosstalk component
and an average phase of the signal light beam, and an optical path
length difference .theta. between the signal light beam and the
reference light beam to carry out an arithmetic operation of
asin(.PSI.-.theta.(t))-b*cos(.PSI.-.theta.(t))
to obtain a reproduction signal; and
obtaining a successive change amount .DELTA..theta. of .theta. and
updating .theta. with the successive variation .DELTA..theta..
Effects of the Invention
According to at least one embodiment, a land/groove recording type
optical recording medium can be satisfactorily reproduced by using
a homodyne detection method. In the present technology, compared to
the method of finding a phase .theta. of the reference light beam
through the low pass filter of the difference signal, a signal with
high responsiveness and stable for a long time is obtained. Note
that the effects of the present technology described herein are not
necessarily limited but may include any effect described in the
present technology.
BRIEF DESCRIPTION OF THE DRAWINGS
FIG. 1 is an explanatory diagram of a cross-sectional structure of
an optical recording medium to be reproduced.
FIG. 2 is an explanatory diagram of a structure of a recording
surface of the optical recording medium to be reproduced.
FIG. 3 is a schematic diagram illustrating a relationship between a
beam spot of a reproduction light beam formed on the recording
surface and a land and a groove.
FIG. 4 is a schematic diagram used for explaining a reproduction
state of the optical recording medium.
FIG. 5 is a schematic diagram illustrating a configuration of an
optical system used in a reproducing apparatus.
FIG. 6 is a block diagram of a signal generating system of the
reproducing apparatus using a conventional phase diversity
method.
FIG. 7 is a schematic diagram for explaining the reproduction state
of the optical recording medium.
FIG. 8 is a schematic diagram for explaining a phase diversity
method.
FIG. 9 is a schematic diagram illustrating an optical system of a
simulation and a graph illustrating a result obtained by a
simulation of a relationship between a track pitch and a jitter
when a step between the land and the groove are set to different
values, respectively.
FIG. 10 is a schematic diagram illustrating an optical system of a
simulation and a graph illustrating a result obtained by a
simulation of a relationship between a track pitch and a jitter
when a step between the land and the groove are set to different
values, respectively.
FIG. 11 is a block diagram of an improved homodyne system signal
generating system.
FIG. 12 is a graph illustrating a result of a simulation of a
relationship between the track pitch and the jitter in the improved
homodyne system signal generating system.
FIG. 13 is a block diagram used for explaining one embodiment of
the present technology.
FIG. 14 is a block diagram of an example of a configuration of a
phase extraction circuit.
FIG. 15 is a graph used for explaining the phase extraction
circuit.
FIG. 16 is a block diagram of a first example of a photoelectric
conversion circuit for forming an RF signal from the output of a
photoelectric conversion element.
FIG. 17 is a block diagram of a second example of the photoelectric
conversion circuit.
FIG. 18 is a block diagram of a third example of the photoelectric
conversion circuit.
FIG. 19 is a block diagram of a fourth example of the photoelectric
conversion circuit.
FIG. 20 is a block diagram of a fifth example of the photoelectric
conversion circuit.
FIG. 21 is a block diagram of a sixth example of the photoelectric
conversion circuit.
FIG. 22 is a block diagram of a seventh example of the
photoelectric conversion circuit.
MODE FOR CARRYING OUT THE INVENTION
The embodiments described below are specific favorable examples of
the present technology and a variety of technologically preferable
limitations is given thereto. However, the scope of the present
technology is not limited to these embodiments unless, in
particular, the description that limits the present technology is
described in the following description.
Note that the description of the present technology is done in the
following order.
<1. Conventional homodyne detection method>
<2. Improved homodyne detection method>
<3. One embodiment>
<4. Example of photoelectric conversion circuit>
<5. Modified Example>
1. Conventional Homodyne Detection Method
Prior to the description of the reproducing method of one
embodiment of the present technology, a conventional homodyne
detection method and an improved homodyne detection method will be
described. Hereinafter, as an example, a homodyne detection method
based on a so-called phase diversity method will be described.
"Optical Recording Medium to be Reproduced"
FIG. 1 illustrates a cross-sectional structural view of an optical
recording medium 1 to be reproduced. The optical recording medium 1
which is rotationally driven is irradiated with a laser beam to
reproduce a recording signal. The optical recording medium 1 is a
so-called write-once type optical recording medium in which
information is recorded by forming, for example, a recording
mark.
As illustrated in FIG. 1, in the optical recording medium 1, a
cover layer 2, a recording layer (reflecting film) 3, and a
substrate 4 are formed in order from an upper layer side. Here, the
"upper layer side" refers to the upper layer side when a surface on
which the laser beam from a reproducing apparatus side is incident
is an upper surface. That is, in this case, the laser beam is
incident on the optical recording medium 1 from a cover layer 2
side.
In the optical recording medium 1, the substrate 4 includes a resin
such as for example, polycarbonate, and an uneven sectional shape
is given on the upper surface side. The substrate 4 is generated by
for example, injection molding using a stamper or the like.
Then, the recording layer 3 is formed on the upper surface side of
the substrate 4 given the uneven shape by sputtering or the like.
Here, the track of the optical recording medium 1 to be reproduced
by the conventional homodyne detection is formed with a normal
track pitch not exceeding an optical limit value. In other words,
the track pitch in the recording layer 3 is set to be larger than
the optical limit value whose theoretical value is represented by
".lamda./NA/2" (.lamda. is a reproduction wavelength and NA is a
numerical aperture of an objective lens).
The cover layer 2 formed on the upper layer side of the recording
layer 3 is formed by, for example, applying an ultraviolet curing
resin by a spin coating method or the like and then performing a
curing treatment by ultraviolet irradiation. The cover layer 2 is
provided for protecting the recording layer 3.
FIG. 2 illustrates a structure of a recording surface of the
optical recording medium 1 to be reproduced. FIG. 2A is a plan view
enlarging a part of the recording surface, and FIG. 2B is a
partially enlarged perspective view of the recording surface. Note
that FIG. 2B illustrates a surface on the side irradiated with a
laser beam for reproduction. In other words, a laser beam for
reproduction is applied from the upper side of the drawing. In the
optical recording medium 1, grooves G and lands L are formed. Here,
in this specification, similarly to a case of BD (Blu-ray Disc:
registered trademark), a side on which the laser beam for
reproduction first comes, that is, a side of a projecting portion
is referred to as the groove G, and a side of a recessed portion is
referred to as the land L.
In the optical recording medium 1 to be reproduced, a mark row is
formed in each of the groove G and the land L. Assuming that the
mark row is a track, a track pitch Tp can be defined as a formation
pitch of the land L and the groove G as illustrated in FIG. 2B. By
setting the track pitch Tp to a narrow pitch exceeding the optical
limit value, information recording density is improved. For
example, assuming that the formation pitch of the grooves G in the
optical recording medium 1 is the same as the track pitch
(formation pitch of mark rows) in the conventional optical
recording medium, the information recording density of the optical
recording medium 1 is increased to nearly twice the conventional
information recording density.
A step (appropriately referred to as a depth) between the land L
and the groove G is represented by d. For example, in a case where
a refractive index of the optical recording medium 1 is n, the
depth d is set to ".lamda./8/n". For example, if the reproduction
wavelength .lamda.=405 nm and n=1.5, a depth d of about 33 nm is
formed.
Here, in the optical recording medium 1, since the formation pitch
between the land L and the groove G exceeds the optical limit
value, a relationship between a beam spot of a reproduction light
beam formed on the recording surface and the land L and the groove
G is as illustrated in FIG. 3, for example.
It is assumed that a tracking servo control of the objective lens
is performed on the groove G or the land L as in the conventional
case. FIG. 3 exemplifies a case where the tracking servo control of
the objective lens is performed on the groove G as a target. In
this case, it is found that the recorded information of two
adjacent lands L coexists with the reproduction signal of the
groove G, the reproduction signal being a servo target.
In other words, in a land/groove recording method, when the track
pitch narrows, crosstalk occurs from the adjacent track. As
illustrated in FIG. 4, when the groove is reproduced, not only a
reproduction signal f(t) of the groove but also a reproduction
signal g(t) of the adjacent land are mixed. When a phase .PHI. of
the groove reproduction signal is equal to 0, the land phase .PSI.
is equal to 4.pi.nd/.lamda. (.lamda. is a wavelength and n is a
refractive index of the substrate of the optical recording medium
1).
"Homodyne Detection Method by Phase Diversity Method"
In the phase diversity method, sets of four signal light beams and
reference light beams which are made such that their phase
differences differ by 90 degrees, respectively, are used.
Specifically, in the phase diversity method, detection is performed
on the sets of the signal light beams and the reference light beams
adjusted so that the phase differences are approximately 0 degree,
approximately 180 degrees, approximately 90 degrees, and
approximately 270 degrees, respectively. Each of these detections
is performed by detecting the light intensity of the light beam in
which the signal light beam and the reference light beam are made
to interfere with each other.
FIG. 5 mainly illustrates the configuration of the optical system
used in the phase diversity method. When the optical recording
medium 1 is loaded in the reproducing apparatus, the optical
recording medium 1 is rotationally driven by a spindle motor. A
laser (semiconductor laser) 10 serving as a laser beam source for
reproduction is provided in the optical system. The laser beam
emitted from the laser 10 is collimated via the collimation lens
11, and then is incident on a polarization beam splitter 13 via a
half-wavelength plate 12.
At this time, it is assumed that the polarization beam splitter 13
is, for example, configured to cause a P polarization to transmit
therethrough and reflect an S polarization. It is assumed that an
attachment angle (a rotation angle around an optical axis within an
incident surface of the laser beam) of the half-wavelength plate 12
is adjusted so that a ratio (that is, a spectral ratio by the
polarization beam splitter 13) of a light beam (P polarization
component) output by transmitting through the polarization beam
splitter 13 and a light beam (S polarization component) reflected
and output is approximately 1:1.
The laser beam reflected by the polarization beam splitter 13
passes through a quarter-wavelength plate 14 and thereafter is
applied so as to condense on the recording layer of the optical
recording medium 1 via an objective lens 15 held by a biaxial
actuator 16.
The biaxial actuator 16 holds the objective lens 15 displaceably in
a focusing direction (a direction in which the objective lens 15
comes into contact with and separates from the optical recording
medium 1) and a tracking direction (a radial direction of the
optical recording medium 1: a direction orthogonal to the focus
direction). The biaxial actuator 16 is provided with a focus coil
and a tracking coil. A focus drive signal FD and a tracking drive
signal TD, which will be described later, are supplied to the focus
coil and the tracking coil, respectively. The objective lens 15 is
displaced in the focus direction and in the tracking direction
according to the focus drive signal FD and the tracking drive
signal TD, respectively.
A reflected light beam from the recording layer of the optical
recording medium 1 is incident on the polarization beam splitter 13
via the objective lens 15 and the quarter-wavelength plate 14. A
polarization direction of the reflected light beam (return light
beam) incident on the polarization beam splitter 13 differs by 90
degrees with respect to a polarization direction of the light beam
(outward light beam) incident from the laser 10 side and reflected
by the polarization beam splitter 13, by the action of the
quarter-wavelength plate 14 and the action at the time of
reflection at the recording layer. In other words, the reflected
light beam is incident on the polarization beam splitter 13 with P
polarization. Therefore, the reflected light beam transmits through
the polarization beam splitter 13. Note that hereinafter, the
reflected light beam reflecting the recording signal of the optical
recording medium 1 that will transmit through the polarization beam
splitter 13 in this way is referred to as a signal light beam.
In FIG. 5, a laser beam (P polarization) emitted from the laser 10
and transmitted through the polarization beam splitter 13 functions
as a reference light beam in the homodyne detection method. The
reference light beam transmitted through the polarization beam
splitter 13 is reflected by a mirror 18 after passing through the
quarter-wavelength plate 17 in the drawing, passes through the
quarter-wavelength plate 17 again, and is incident on the
polarization beam splitter 13.
Here, as described above, the reference light beam (return light
beam) to be incident on the polarization beam splitter 13 differs
by 90 degrees from the reference light beam as the outward light
beam, by the action of the quarter-wavelength plate 17 and the
action at the time of reflection on the mirror 18 (that is, S
polarization). Accordingly, the reference light beam as the return
light beam is reflected by the polarization beam splitter 13.
In FIG. 5, the reference light beam reflected by the polarization
beam splitter 13 as described above is indicated by a broken line
arrow. In FIG. 5, a signal light beam transmitted through the
polarization beam splitter 13 is indicated by a solid arrow. The
signal light beam and the reference light beam are emitted in the
same direction by the polarization beam splitter 13 in a state of
being superimposed. Specifically, in this case, the signal light
beam and the reference light beam are emitted in the same direction
while being superimposed so that optical axes of the signal light
beam and the reference light beam are aligned. Here, the reference
light beam is so-called coherent light.
The superimposed light beam of the signal light beam and the
reference light beam output from the polarization beam splitter 13
is incident on a half beam splitter 19. The half beam splitter 19
splits an incident light beam into a reflected light beam and a
transmitted light beam at a ratio of approximately 1:1.
The superimposed light beam of the signal light beam and the
reference light beam transmitted through the half beam splitter 19
is incident on a polarization beam splitter 21 via a
half-wavelength plate 20. On the other hand, the superimposed light
beam of the signal light beam and the reference light beam
reflected from the half beam splitter 19 is incident on a
polarization beam splitter 23 via a quarter-wavelength plate
22.
The half-wavelength plate 20 and the quarter-wavelength plate 22
are capable of rotating a polarization plane. Accordingly, by
combining the half-wavelength plate 20 and the polarization beam
splitter 21, it is possible to adjust the ratio of the quantity of
a light beam branched by the polarization beam splitter 21.
Similarly, the ratio of the quantity of a light beam branched by
the polarization beam splitter 23 can be adjusted by the
quarter-wavelength plate 22.
The ratio of the quantity of a light beam can be adjusted so that
the amount of a light beam branched by each of the polarization
beam splitter 21 and 23 is approximately 1:1. A light beam
reflected by the polarization beam splitter 21 is incident on a
light detection unit 24, and the light beam transmitted through the
polarization beam splitter 21 is incident on a light detection unit
25. A light beam reflected by the polarization beam splitter 23 is
incident on a light detection unit 26, and the light beam
transmitted through the polarization beam splitter 23 is incident
on a light detection unit 27.
A light receiving signal output from the light detection unit 24 is
denoted by I, a light receiving signal output from the light
detection unit 25 is denoted by J, a light receiving signal output
from the light detection unit 26 is denoted by L, and a light
receiving signal output from the light detection unit 27 is denoted
by K.
These light receiving signals I to L are supplied to subtraction
circuits 31a and 31b. The light receiving signals I and J are
supplied to the subtraction circuit 31a, the subtraction circuit
31a generates a difference signal a of (a=I-J), and the subtraction
circuit 31b generates a differential signal b of (b=K-L).
As illustrated in FIG. 6, the differential signals a and b
described above are supplied to an arithmetic circuit 32. The
arithmetic circuit 32 includes delay circuits 33a and 33b,
multiplying circuits 34a and 34b, low pass filters 35a and 35b,
offset (.PHI.) setting circuits 36a and 36b, and an addition
circuit 37. The delay circuit 33a has a delay time equal to a delay
amount generated in the low pass filter 35a and the offset (.PHI.)
setting circuit 36a. The delay circuit 33b has a delay time equal
to a delay amount generated in the low pass filter 35b and the
offset (.PHI.) setting circuit 36b. The output of the multiplying
circuit 34a and the output of the multiplying circuit 34b are
supplied to the addition circuit 37. A reproduction signal is taken
out to the output of the addition circuit 37.
As described below, the above-described reproducing apparatus can
obtain a reproduction signal which is not affected by the component
of a phase shift (.theta.(t)) of the reference light beam due to
surface blur of the optical recording medium 1 or the like.
The light receiving signals I to L are expressed by the following
numerical expressions. The meaning of each term in the expression
is illustrated below.
R: a reference light beam component
A: a reflection component of a mirror surface (land portion) formed
on the recording surface of the optical recording medium
f: a modulated component (taking a positive/negative value)
according to presence/absence of recording mark
t: sampling time
.PHI.: a phase difference between a mark to be read and an average
phase of a signal light beam. A value estimated and set by a
user.
.theta.: a difference in an optical path length between a signal
light beam and a reference light beam (mainly caused by surface
blur of the optical recording medium 1)
As illustrated in FIG. 7, when the objective lens 15 and a signal
surface of the optical recording medium 1 change due to surface
blur, the optical path length of the signal light beam changes. On
the other hand, since the reference light beam is reflected by the
mirror 18, the optical path length does not change. As a result,
the phase difference between the signal light beam and the
reference light beam is shifted from the set value. The component
of this phase shift is .theta.(t). [Expression 1]
4I=|A+f(t)e.sup.i.PHI.+Re.sup.i.PHI.|.sup.2=(A+f cos .PHI.+R cos
.theta.).sup.2+(f sin .PHI.+R sin .theta.).sup.2 (1) [Expression 2]
4J=|A+f(t)e.sup.i.PHI.-Re.sup.i.PHI.|.sup.2=(A+f cos .PHI.-R cos
.theta.).sup.2+(f sin .PHI.-R sin .theta.).sup.2 (2) [Expression 3]
4K=|A+f(t)e.sup.i.PHI.+Re.sup.i.PHI.|.sup.2=(A+f cos .PHI.-R cos
.theta.).sup.2+(f sin .PHI.+R sin .theta.).sup.2 (3) [Expression 4]
4I=|A+f(t)e.sup.i.PHI.-Re.sup.i.PHI.|.sup.2=(A+f cos .PHI.+R cos
.theta.).sup.2+(f sin .PHI.-R sin .theta.).sup.2 (4)
The difference signal a (=I-J) of the subtraction circuit 31a and
the difference signal b (=K-L) of the subtraction circuit 31b are
as illustrated in the following expressions. [Expression 5]
a=I-J=(A+f cos .PHI.)R cos .theta.+f sin .PHI.R sin .theta.=AR cos
.theta.+fR cos(.PHI.-.theta.) (5) [Expression 6] b=K-L=-(A+f cos
.PHI.)R sin .theta.+f sin .theta.+f sin .PHI.R cos .theta.=-AR sin
.theta.+fR sin(.PHI.-.theta.) (6)
As illustrated in FIG. 8A, even in ordinary detection without using
homodyne detection, the DC component of the reproduction signal
appears in accordance with the mirror portion of the background. In
a case of homodyne detection, as illustrated in FIG. 8B, the DC
component corresponding to the mirror portion undulates according
to a phase .theta. corresponding to a difference in a reference
light beam optical path length.
In order to obtain the phase .theta., the difference signals a and
b illustrated in FIG. 8B are supplied to the low pass filters 35a
and 35b, respectively. As illustrated in FIG. 8C, cos .theta.(t)
and sin .theta.(t) can be obtained by the low pass filters 35a and
35b. In other words, in the numerical expressions (5) and (6),
since f is assumed to be a modulation component (taking a
positive/negative value) according to presence/absence of the
recording mark, it is considered that a term multiplied by the
function f disappears and the terms of sin .theta. and cos .theta.
remain.
Since (tan .theta.=sin .theta./cos .theta.), .theta. is obtained by
(arctan .theta.=.theta.), .PHI. (offset) is set, the multiplying
circuit 34a multiplies (cos(#-.theta.(t)) by a, and the multiplying
circuit 34b multiplies (sin (.PHI.-.theta.(t)) by b. Then, the
multiplication output is added by the addition circuit 37. The
reproduction signal obtained from the addition circuit 37 is
represented by the following expression. [Expression 7]
acos(.PHI.-.theta.(t))+bsin(.PHI.-.theta.(t))=f(t)R+AR cos .PHI.
(7)
As can be seen from this numerical expression, in the reproduction
signal, the component of .theta.(t) disappears and the reproduction
signal becomes a stable signal. Note that as the homodyne detection
system, there is a method of performing the position control of the
mirror 18 to cancel the phase difference between the signal light
beam and the reference light beam caused by surface blur. However,
according to the phase diversity method, such a configuration for
position control of the mirror 18 can be omitted. Moreover, it can
be seen that a reproduction result obtained by amplifying the
component of the signal light beam with the component of the
reference light beam can be obtained. In other words, the recording
signal of the optical recording medium 1 is amplified and detected,
and improvement of SNR is attained in this respect. Note that the
term of the phase diversity method means a method of obtaining a
reproduction signal by calculating the square root of the square
sum (a2+b2) or the square sum of the differential signals a and b.
In the present specification, as described above, the term of the
phase diversity method is used even for the arithmetic operation in
which (cos(.PHI.-.theta.(t)) is multiplied by a and in the
multiplying circuit 34b, (sin(.PHI.-.theta.(t)) is multiplied by
b.
It is assumed that the optical recording medium of the land/groove
recording as described above is reproduced by the optical system
illustrated in FIG. 9A. The result obtained by a simulation of
jitter of the reproduction signal (groove reproduction signal or
land reproduction signal) when a track pitch Tp is changed is
illustrated in the graph of FIG. 9B. Note that jitter is one of
indicators indicating playback performance, and indexes other than
jitter may be used.
As illustrated in FIG. 9A, a laser beam from a laser diode 41
passes through a lens 42, a polarization beam splitter 43, and an
objective lens 44 and is applied onto the signal surface of the
optical recording medium 1. A reflected light beam from the signal
surface is reflected by the polarization beam splitter 43 and is
supplied to a light detection unit 46 via a lens 45. A reproduction
signal is obtained from the light detection unit 46. A reproducing
optical system illustrated in FIG. 9A does not use the
above-described homodyne detection.
The simulation is performed under the following calculation
conditions. Note that it is assumed that there is no surface blur,
and a reproducing method that reduces crosstalk between tracks is
used.
.lamda.=405 nm, NA=0.85, rim=65%/65%, and groove duty=50%
Slope=90 degrees, mark reflectivity=0%, mark width=0.9 Tp, and
linear density=25 GB constant
For each of (Mrr (meaning a mirror, d=0), (d=0.125.lamda.),
(d=0.15.lamda.), and (d=0.175.lamda.)), the graph illustrated in
FIG. 9B illustrates the change of a jitter value with respect to
Tp. For example, in (Tp=0.22), jitter can be reduced with respect
to the depth of a groove other than a mirror. Moreover, even if the
depth of the groove is different, the jitter change can be made
almost a similar.
FIG. 10 illustrates a simulation result when the optical recording
medium 1 of land/groove recording is reproduced by using homodyne
detection. As illustrated in FIG. 10A, a mirror 47 is provided, and
the reflected light beam (signal light beam) from the optical
recording medium 1 and the reflected light beam (reference light
beam) from the mirror 47 are supplied to the light detection unit
46 via the lens 45.
FIG. 10B illustrates a simulation result when the optical system
illustrated in FIG. 10A is used. Calculation conditions of the
simulation are similar to those in FIG. 9B. For each of (Mrr
(meaning a mirror, d=0), (d=0.1.lamda.),
(d=0.125.lamda.=.lamda./8), (d=0.15.lamda.), and (d=0.175.lamda.)),
the graph illustrated in FIG. 10B illustrates the change of a
jitter value with respect to Tp.
For example, in (Tp=0.15), jitter can be reduced compared with the
mirror. However, the change of a jitter value varies depending on
the value of depth d. In other words, in a case of
(d=0.125.lamda.=.lamda./8), jitter can be greatly improved, whereas
in the case of (d=0.175.lamda.), jitter is too large. Moreover, the
values of jitter in cases of (d=0.1.lamda.) and (d=0.15.lamda.) are
not sufficiently satisfactory. In a case of d=.lamda./8, since a
phase difference of 90 degrees can be generated between the groove
reproduction signal and the land reproduction signal, crosstalk can
be reduced and jitter can be improved.
As described above, restriction on design of the optical recording
medium 1 arises that satisfactory reproduction performance can be
obtained only for a case of the depth d for a specific groove.
Moreover, the value of d=.lamda./8 is a relatively large value,
which is not preferable in terms of recording marks on the land
between the grooves. Moreover, in a case where d is large, it is
difficult to make a surface of a wall of a step steep without
inclination when molding the optical disk. Accordingly, it is
preferable that the value of d is not limited to (.lamda./8).
2. Improved Homodyne Detection Method
In order to improve this point, a reproducing optical system
similar to that illustrated in FIG. 5 is used and a reproduction
signal generating circuit similar to that illustrated in FIG. 6 is
used. A difference signal formed from the light receiving signals I
to L output from the light detection units 24 to 27 in FIG. 5 is
supplied to the reproduction signal generating circuit having a
configuration as illustrated in FIG. 11.
The reproduction signal generating circuit includes the subtraction
circuits 31a and 31b and an arithmetic circuit 40. The light
receiving signals I and J are supplied to the subtraction circuit
31a, the subtraction circuit 31a generates a difference signal a of
(a=I-J), and the arithmetic circuit 31b generates a differential
signal b of (b=K-L). The difference signal a of the subtraction
circuit 31a and the difference signal b of the subtraction circuit
31b are supplied to the arithmetic circuit 40.
The arithmetic circuit 40 has delay circuits 33a and 33b,
multiplying circuits 34a and 34b, low pass filters 35a and 35b,
offset (.PSI.) setting circuits 39a and 39b, and a subtraction
circuit 40. The delay circuit 33a has a delay time equal to a delay
amount generated in the low pass filter 35a and the offset (.PSI.)
setting circuit 39a. The delay circuit 33b has a delay time equal
to a delay amount generated in the low pass filter 35b and the
offset (.PSI.) setting circuit 39b. The output of the multiplying
circuit 34a and the output of the multiplying circuit 34b are
supplied to a subtraction circuit 50. A reproduction signal is
taken out to the output of the subtraction circuit 50.
As described below, in the offset (.PSI.) setting circuits 39a and
39b, a value (.PSI.) corresponding to the phase difference between
a crosstalk component and an average phase of the signal light beam
is estimated by the user and set as a fixed value. For example, a
step between the groove G and the land L, that is, a phase offset
according to the depth d is set. Since the value of the depth d of
the optical recording medium 1 to be reproduced is known
beforehand, it is possible to set the offset .PSI..
As described below, in the above-described homodyne system, a
reproduction signal which is free from the influence of the
component of the phase shift (.theta.(t)) of the reference light
beam due to surface blur of the optical recording medium 1 or the
like and in which an inter-track crosstalk has been removed is
obtained. As described with reference to FIGS. 3 and 4, in the
land/groove recording method, when the track pitch narrows,
crosstalk occurs from the adjacent track. As illustrated in FIG. 4,
when the groove is reproduced, not only the reproduction signal
f(t) of the groove but also the reproduction signal g(t) of the
adjacent land are mixed. When a phase .PHI. of the groove
reproduction signal is equal to 0, the land phase .PSI. is equal to
4.pi.nd/.lamda. (.lamda. is a wavelength and n is a refractive
index of the substrate of the optical recording medium 1).
The light receiving signals I to L are obtained using the
reproducing optical system illustrated in FIG. 5. The meanings of
each term in the expression are described below as in the
above-described numerical expression.
R: a reference light beam component
A: a reflection component of a mirror surface (land portion) formed
on the recording surface of the optical recording medium
f: a modulated component (taking a positive/negative value)
according to presence/absence of recording mark
g: crosstalk component from adjacent track
t: sampling time
.PHI.: a phase difference between a mark to be read and an average
phase of a signal light beam. A value estimated and set by a
user.
.theta.: a difference in an optical path length between a signal
light beam and a reference light beam (mainly caused by surface
blur of the optical recording medium 1)
.PSI.: a phase difference between the crosstalk component and the
average phase of the signal light beam. A value estimated and set
by a user. [Expression 8]
4I=|A+f(t)e.sup.i.PHI.+g(t)e.sup.i.psi.+Re.sup.i.theta.|.sup.2=(A+f
cos .PHI.+g cos .psi.+R cos .theta.).sup.2+(f sin .PHI.+g cos
.psi.+R sin .theta.).sup.2 (8) [Expression 9]
4J=|A+f(t)e.sup.l.PHI.+g(t)e.sup.l.psi.-Re.sup.i.theta.|.sup.2=(A+f
cos .PHI.+g cos .psi.-R cos .theta.).sup.2+(f sin .PHI.+g cos
.psi.-R sin .theta.).sup.2 (9) [Expression 10]
4K=|A+f(t)e.sup.i.PHI.+g(t)e.sup.i.psi.+iRe.sup.i.theta.|.sup.2=(A+f
cos .PHI.+g cos .psi.+R sin .theta.).sup.2+(f sin .PHI.+g sin
.psi.+R cos .theta.).sup.2 (10) [Expression 11]
4L=|A+f(t)e.sup.i.PHI.+g(t)e.sup.i.psi.-iRe.sup.i.theta.|.sup.2=(A+f
cos .PHI.+g cos .psi.+R sin .theta.).sup.2+(f sin .PHI.+g sin
.psi.-R cos .theta.).sup.2 (11)
Moreover, an arithmetic operation is performed using the
reproduction signal generating circuit illustrated in FIG. 11. The
difference signal a (=I-J) of the subtraction circuit 31a and the
difference signal b (=K-L) of the subtraction circuit 31b are as
illustrated in the following expressions. [Expression 12]
a=I-J=(A+f cos .PHI.+g cos .psi.)R cos .theta.+(f sin .PHI.+g sin
.psi.)R sin .theta.=AR cos .theta.+fR cos(.PHI.-.theta.)+gR
cos(.psi.-.theta.) (12) [Expression 13] b=K-L=-(A+f cos .PHI.+g cos
.psi.)R sin .theta.+(f sin .PHI.+g sin .psi.)R cos .theta.=AR sin
.theta.+fR sin(.PHI.-.theta.)+gR sin(.psi.-.theta.) (13)
As described above, cos .theta.(t) and sin .theta.(t) are obtained
by the low pass filters 35a and 35b. In other words, in the
numerical expressions (12) and (13), since f is assumed to be a
modulation component (taking a positive/negative value) according
to presence/absence of the recording mark, and g is a crosstalk
component from the adjacent track, it is considered that a term
multiplied by the functions f and g disappears and the terms of sin
.theta. and cos .theta. remain. Since (tan .theta.=sin .theta./cos
.theta.), .theta. is obtained by (arctan .theta.=.theta.), .PSI.
(offset) is set by the offset (.PSI.) setting circuits 39a and 39b,
the multiplying circuit 34a multiplies (sin(.PSI.-.theta.(t)) by a,
and the multiplying circuit 34b multiplies (cos(.PSI.-.theta.(t))
by b. Then, the subtraction circuit 40 combines these
multiplication outputs. The reproduction signal obtained from the
subtraction circuit 40 is represented by the following expression.
[Expression 14]
a.times.sin(.psi.-.theta.(t))-b.times.cos(.psi.-.theta.(t))=AR cos
.theta. sin(.psi.-.theta.)+fR
cos(.PHI.-.theta.)sin(.psi.-.theta.)-fR
sin(.PHI.-.theta.)cos(.psi.-.theta.)=f(t)R sin(.psi.-.theta.)+AR
sin .psi. (14)
As illustrated in the expression (14), in the reproduction signal,
the component of .theta.(t) disappears and the reproduction signal
becomes a stable signal. In addition, the reproduction signal g(t)
of the adjacent track is not included in the reproduction signal,
and an inter-track crosstalk is eliminated.
FIG. 12 illustrates a simulation result when an optical system
similar to the optical system illustrated in FIG. 10A is used.
Calculation conditions of the simulation are similar to those in
FIG. 9B and FIG. 10B. For each of (Mrr (meaning a mirror, d=0),
(d=0.1.lamda.), (d=0.125.lamda.=.lamda./8), (d=0.15.lamda.), and
(d=0.175.lamda.)), the graph illustrated in FIG. 12 illustrates the
change of a jitter value with respect to Tp.
As can be seen from the graph of FIG. 12, it is possible to reduce
jitter with respect to all the values of d except the mirror. In
the case of FIG. 10B described above, jitter can be greatly
improved only in the case of (d=0.125.lamda.=.lamda./8), whereas in
the improved homodyne system, even if d is another value, jitter
can be greatly improved.
3. One Embodiment
In the above-described improved homodyne system, the influence of
the shift of the phase difference .theta. between the signal light
beam and the reference light beam can be eliminated, and moreover,
crosstalk can be eliminated by previously setting the offset .PSI.
according to the phase difference between the crosstalk component
and the average phase of the signal light beam. Therefore, .theta.
corresponding to a difference in an optical path length between the
signal light beam and the reference light beam is obtained by the
low pass filters 35a and 35b. However, in the case of the low pass
filter, it is difficult to sufficiently remove a noise component,
and there is also a possibility that a signal component is
removed.
The present technology has been considered in consideration of such
points. The present technology eliminates the influence of the
shift of the phase difference .theta. between the signal light beam
and the reference light beam without using a low pass filter. In
other words, the present technology suppresses deterioration of
signal quality due to fluctuation (perturbation factor) by using an
extracted phase fluctuation component. For the difference signals a
and b, an arithmetic operation is performed using the extracted
phase fluctuation component. As a result, signals represented by
the following expressions (15) and (16) can be independently read.
[Expression 15]
a.times.sin(.psi.-.theta.(t))-b.times.cos(.psi.-.theta.(t)) (15)
[Expression 16]
a.times.sin(.PHI.-.theta.(t))-b.times.cos(.PHI.-.theta.(t))
(16)
FIG. 13 illustrates a configuration example of one embodiment. The
difference signals a and b are supplied to a phase (.theta.)
extraction circuit 71, and the phase is extracted. Offset setting
circuits 72 and 73 are provided, and respectively output offsets
.PHI. and .PSI. set corresponding to an optical disc to be
reproduced. As described above, .PHI. is a phase difference between
the mark to be read and the average phase of the signal light beam,
and .PSI. is the phase difference between the crosstalk component
and the average phase of the signal light beam. These offsets are
the values estimated and set by the user.
The output of the phase extraction circuit 71 and the output of the
offset setting circuit 72 are supplied to a subtraction circuit 74,
and the phase of (.PSI.-.theta.) is obtained from the subtraction
circuit 74. Signal generating circuits 76 and 77 respectively
generate a sine wave and a cosine wave synchronized with the phase
of (.PSI.-.theta.). The difference signal a, and the sine wave from
the signal generating circuit 76 are supplied to a multiplying
circuit 78, and the output signal of the multiplying circuit 78 is
supplied to a subtraction circuit 80. The difference signal b, and
the cosine wave from the signal generating circuit 77 are supplied
to a multiplying circuit 79, and the output signal of the
multiplying circuit 79 is supplied to the subtraction circuit 80.
The reproduction signal represented by the expression (15) is taken
out for the output of the subtraction circuit 80.
The output of the phase extraction circuit 71 and the output of the
offset setting circuit 73 are supplied to a subtraction circuit 75,
and the phase of (.PHI.-.theta.) is obtained from the subtraction
circuit 75. The signal generating circuits 81 and 82 respectively
generate a sine wave and a cosine wave synchronized with the phase
of (.PHI.-.theta.). The difference signal a, and the sine wave from
the signal generating circuit 81 are supplied to a multiplying
circuit 83, and the output signal of the multiplying circuit 83 is
supplied to a subtraction circuit 85. The difference signal b, and
the cosine wave from the signal generating circuit 82 are supplied
to a multiplying circuit 84, and the output signal of the
multiplying circuit 84 is supplied to the subtraction circuit 85.
The reproduction signal represented by the expression (16) is taken
out for the output of the subtraction circuit 85.
In the difference signals a and b, since a portion to which f and g
are applied is an AC component, the AC component becomes 0 by
performing integration, and only a DC component remains. In other
words, a=AR cos .theta., and b=-AR sin .theta. remain. On the other
hand, when .theta. is known, if the following arithmetic operation
is performed on the difference signals a and b, the DC component
becomes 0. a sin .theta.+b cos .theta..fwdarw.0
In order to set the value of (.DELTA..theta.) to 0 by changing
.theta. when the above expression is not 0 at .theta.(t) of the
present time, the relationship of the following expression is
necessary. a sin(.theta.+.DELTA..theta.)+b
cos(.theta.+.DELTA..theta.)=a.DELTA..theta. cos .theta.+a sin
.theta.+b cos .theta.-b.DELTA..theta. sin .theta.=0
A successive change amount .DELTA..theta. of .theta. is obtained by
using the following sequential expression (Expression 17). The
phase .theta. is obtained by updating .theta. by the successive
change amount .DELTA..theta.. In other words, a relationships of
.theta.t+1=.theta.t+.DELTA..theta.t holds. In Expression 17, .eta.
is a learning coefficient, F is a function, for example, F (x) is
x, sin(x), a tan (x), tan h(x), and the like.
.times..times..DELTA..times..times..theta..eta..function..times..times..t-
imes..theta..times..times..times..theta..times..times..times..theta..times-
..times..times..times..times..theta. ##EQU00001##
An example of the phase extraction circuit 71 is illustrated in
FIG. 14. The phase extraction circuit 71 extracts the phase .theta.
by a successive phase detection method. The difference signals a
and b are supplied to an arithmetic circuit 91, and the arithmetic
operation of the above-described expression (Expression 17) is
performed. The output signal of the arithmetic circuit 91 is
supplied to a coefficient multiplying circuit 92 and then
multiplied by a learning coefficient .eta..
The output of the coefficient multiplying circuit 92 is supplied to
an addition circuit 93. The obtained
(.theta.t+1=.DELTA..theta.t+.theta.t) appears at the output of the
addition circuit 93. The output of the addition circuit 93 is taken
out as .theta.t+1 and then supplied to the arithmetic circuit 91
and the addition circuit 93 via a delay circuit 94 of one sample
period T.
An example of simulation for one embodiment of the present
technology will be described. The simulation conditions are as
follows.
In the sequential expression (Expression 17), it is defined that
F=1 and .eta.=0.007.
Disk space: 33.4 GB
Tp=0.16 .mu.m (each land and groove)
Groove depth: .lamda./8
Mark reflectivity: 0.3 (no phase)
PR(12221)
Evaluation index: i-MLSE
FIG. 15 illustrates a simulation result. FIG. 15A illustrates the
difference signals a and b and the waveforms of the following
signals represented by the above-mentioned expression (15).
(a.times.sin(.PSI.-.theta.(t))-(b.times.cos(.PSI.-.theta.(t))
FIG. 15B illustrates the change of .theta.. FIG. 15C illustrates
the value of i-MLSE before applying sequential phase correction of
the present technology (in other words, the difference signals a
and b) and the value of i-MLSE after applying the sequential phase
correction of the present technology (in other words, the waveform
on the lowermost side in FIG. 15A). A maximum likelihood sequence
error (MLSE) is obtained by calculating an index corresponding to
an error probability by using a difference in level of an actual
signal with respect to a target level set using Viterbi detected
data. Since smaller values of i-MLSE are better reproduction, it
can be seen that good reproduction is possible by applying the
present technology.
According to the present technology described above, compared to
the method of detecting .theta. through the low pass filter of the
difference signals a and b, there is an advantage that stable
signal processing can be performed.
4. Example of Photoelectric Conversion Circuit
In the above-described homodyne detection method, as described with
reference to FIG. 8B in the output signals of the light receiving
elements, for example, in the difference signals a and b, there is
a problem that the dynamic range of the photoelectric conversion
circuit is consumed and the SNR of the modulation component is
lowered because the level of an unmodulated low frequency component
is large. If the photoelectric conversion circuit is AC-coupled,
the SNR of the modulation component can be secured, but there is a
problem that part of the information of the reference light beam
phase is lost.
The photoelectric conversion circuit described below can solve such
a problem. In other words, after photoelectrically converting the
reproduction light beam with two light receiving elements for AC
and DC, the original reproduction signal is restored by adding an
AC component and a DC component by an adder capable of securing a
wide dynamic range and a high SNR. The photoelectric conversion
circuit can be applied to each of the light detection units 24 to
27 in the optical system illustrated in FIG. 5, for example.
With such a configuration, the entire dynamic range of a light
receiving circuit can be occupied by modulation components, and
degradation of SNR can be prevented. Furthermore, by adding signals
from the two photoelectric conversion circuits with an adder
capable of securing the dynamic range, it is possible to narrow the
band of the signal input to the adder from a DC coupling side, and
to reduce the noise of the signal after addition.
FIG. 16 illustrates a first example of the photoelectric conversion
circuit. The reproduction light beam is split by a beam splitter
101 and a mirror 102. The reproduction light beam from the mirror
102 is incident on a photoelectric conversion circuit 103 in which
a photodetector (light receiving element) 104 and a high pass
filter 105 are connected in series, and is converted into an
electric signal. A low frequency component is removed by the high
pass filter 105, and a high frequency component from a high pass
filter 105 is converted into a digital signal by an A/D converter
106. The digital signal is supplied to an adder 107. A cutoff
frequency of the high pass filter 105 is selected to be a frequency
that can eliminate low-frequency fluctuation of the reproduction
signal.
The reproduction light beam from the beam splitter 101 is incident
on a photodetector 109 of a photoelectric conversion circuit 108,
and a reproduction signal is obtained. The reproduction signal is
supplied to a low pass filter 110. The low frequency component
separated by the low pass filter 110 is converted into a digital
signal in an A/D converter 111. The output of the A/D converter 111
is supplied to a coefficient multiplier 112. The output of the
coefficient multiplier 112 is supplied to the adder 107. The output
of the coefficient multiplier 112 is then added to a high frequency
component by the adder 107. From the adder 107, a low frequency
component and a high frequency component are obtained.
The frequency characteristic of the low pass filter 110 is
complementary to the frequency characteristic of the high pass
filter 105. In other words, assuming that the transfer function of
the high pass filter 105 is H(f) and the transfer function of the
low pass filter 110 is G(f), the relationship is H(f)=1-G(f), and
after addition, a gain is kept constant within a predetermined
band. The first example of the photoelectric conversion circuit
illustrated in FIG. 16 has an advantage that a reference light beam
servo is not required.
FIG. 17 illustrates a second example of the photoelectric
conversion circuit. In the second example, a reference light beam
servo 113 is added to the configuration of the above-described
first example. In other words, an electric signal from the
photoelectric conversion circuit 108 is supplied to the low pass
filter 110 and the reference light beam servo 113. The reference
light beam servo 113 physically makes the optical path length of
the reference light beam changeable. For example, the reference
light beam servo 113 has a configuration in which the position of
the mirror 18 (see FIG. 5) in the optical path of the reference
light beam is moved by a control signal to make the optical path
length of the reference light beam changeable. The time variation
of the phase difference between the signal light beam and the
reference light beam can be removed by the reference light beam
servo.
The output of the low pass filter 110 is a residual component of
the low frequency component. The residual component is digitized by
the A/D converter 111, multiplied by coefficients by coefficient
multiplier 112, and supplied to the adder 107. In the adder 107,
the low frequency component is added to the high frequency
component. From the adder 107, a residual component of the low
frequency component is obtained.
FIG. 18 illustrates a third example of the photoelectric conversion
circuit. In the third example, a fixed DC value is added to the
configuration of the second example in the adder 107. Since the
reference light beam servo 113 is provided, the time variation of
the phase difference between the signal light beam and the
reference light beam can be removed, so that the fixed DC value is
added in the adder 107. The fixed DC value is a value previously
obtained.
FIG. 19 illustrates a fourth example of the photoelectric
conversion circuit. The reference light beam servo 113 is provided.
The output of the photodetector 104 of the photoelectric conversion
circuit 103 is supplied to a subtractor 114, and the fixed DC value
is subtracted from the output of the photodetector 104. The fixed
DC value is a value previously obtained. The output of the
subtractor 114 is supplied to the A/D converter 106. A low
frequency component is removed beforehand at the input side of the
A/D converter 106.
FIG. 20 illustrates a fifth example of the photoelectric conversion
circuit. The reference light beam servo 113 is provided. To the
reference light beam servo 113, an output signal of the
photodetector 104 and a target phase (for example, a DC value
corresponding to the target phase) 115 are given. The reference
light beam servo 113 makes the phase of the output signal of the
photodetector 104 coincide with the target phase.
The output of the photodetector 104 is supplied to the subtractor
114. A voltage (DC value) of a level corresponding to the target
phase is formed by a phase-voltage level modulation circuit 116.
This DC value is supplied to the subtractor 114 and then subtracted
from the output signal of the photodetector 104. The signal
obtained at the output of the subtractor 114 has an extremely low
frequency component. Then, the voltage (DC value) of the level
corresponding to the target phase is added in the adder 107. Note
that in a case where the DC value is added by the adder 107, a
level correction coefficient for the DC value may be multiplied. It
is possible to avoid the limitation of a dynamic range of an analog
circuit in the path from the photodetector 104 to the A/D converter
106. Furthermore, the configuration of FIG. 20 has an advantage
that it is not necessary to divide the optical path. In a case of a
configuration where the optical path division is unnecessary, the
difference signal a or b may be used instead of the output signal
of the photodetector 104.
FIG. 21 illustrates a sixth example of the photoelectric conversion
circuit. In this example, as in the fifth example, there is no need
to divide the optical path, and the reference light beam servo 113
is provided. A DC value 118 is supplied to the subtractor 114 of
the photoelectric conversion circuit 103. The output of the
subtractor 114 is converted into a digital signal by the A/D
converter 106, and supplied to the adder 107. A value obtained by
digitizing the DC value 118 by an A/D converter 117 is supplied to
the adder 107. Note that in a case where the DC value is added by
the adder 107, a level correction coefficient for the DC value may
be multiplied.
FIG. 22 illustrates a seventh example of the photoelectric
conversion circuit. In this example, as in the fifth example, there
is no need to divide the optical path, and the reference light beam
servo 113 is provided. The target phase 115 given to the reference
light beam servo 113 is supplied to the phase-voltage level
conversion circuit 116. A voltage corresponding to the target phase
is formed by the phase-voltage level conversion circuit 116, and
this voltage is supplied to the adder 107. Note that in a case
where the DC value is added by the adder 107, a level correction
coefficient for the DC value may be multiplied.
5. Modified Example
The embodiments according to the present technology have been
specifically described above. However, the present technology is
not limited to the aforementioned respective embodiments and
various modified examples based on the technological spirit of the
present technology can be made. For example, the wavelength of the
laser beam source may be other than 405 nm.
Moreover, the reproducing optical system is not limited to the
configuration illustrated in FIG. 5, but for example, a homodyne
detection optical system may be used to obtain four kinds of the
light receiving signals I to L. The homodyne detection optical
system has a Wollaston prism and is capable of generating light
beams having respective phase differences of 0 degrees, 90 degrees,
180 degrees, and 270 degrees.
Furthermore, the configurations, methods, processes, shapes,
materials, numerical values, and the like exemplified in the
above-described embodiments can be mutually combined without
departing from the spirit of the present technology.
Note that the present technology can also be configured as
follows.
(1)
A reproducing apparatus including:
an optical system that irradiates a recording medium on which
signals are each recorded on a land and a groove with a light beam
emitted from a light source to obtain a signal light beam
reflecting each of the recording signals of the land and the
groove, generates a reference light beam from the light beam
emitted from the light source, and generates a set of a first
signal light beam and a first reference light beam which give a
phase difference of approximately 0.degree. to a superimposed light
beam obtained by superimposing the signal light beam and the
reference light beam, a set of a second signal light beam and a
second reference light beam which give a phase difference of
approximately 180.degree. to the superimposed light beam, a set of
a third signal light beam and a third reference light beam which
give a phase difference of approximately 90.degree. to the
superimposed light beam, and a set of a fourth signal light beam
and a fourth reference light beam which give a phase difference of
approximately 270.degree. to the superimposed light beam;
a light receiving unit that uses a first light receiving element to
receive the set of the first light beam and the first reference
light beam, a second light receiving element to receive the set of
the second signal light beam and the second reference light beam, a
third light receiving element to receive the set of the third
signal light beam and the reference, and a fourth light receiving
element to receive the set of the fourth signal light beam and the
fourth reference light beam;
a reproduction signal generating circuit that calculates a first
difference signal a which is a difference between a first light
receiving signal obtained by the first light receiving element and
a second light receiving signal obtained by the second light
receiving element, and a second difference signal b which is a
difference between a third light receiving signal obtained by the
third light receiving element and a fourth light receiving signal
obtained by the fourth light receiving element, and
uses the first difference signal a, the second difference signal b,
a phase difference .PSI. between a crosstalk component and an
average phase of the signal light beam, and an optical path length
difference .theta. between the signal light beam and the reference
light beam to carry out an arithmetic operation of
asin(.PSI.-.theta.(t))-bcos(.PSI.-.theta.(t))
to obtain a reproduction signal; and a phase extraction circuit
that obtains a successive change amount .DELTA..theta. of .theta.
and updates .theta. with the successive variation
.DELTA..theta..
(2)
The reproducing apparatus according to claim 1, in which a
successive change amount .DELTA..theta.t is obtained by a following
expression when .theta.t+1=.theta.t+.DELTA..theta.t.
.times..times..times..DELTA..times..times..theta..eta..function..times..t-
imes..times..theta..times..times..times..theta..times..times..times..theta-
..times..times..times..times..times..theta. ##EQU00002##
The reproducing apparatus according to (1) or (2), in which the
reference light beam is generated by reflecting a light beam
emitted from the light source by a mirror.
(4)
The reproducing apparatus according to any one of (1) to (3), in
which the phase offset is assumed to be approximately equal to
(|.PSI.|=4.pi.nd/.lamda.) (n is a refractive index, d is a step
between the land and the groove, and .lamda. is a wavelength of a
light beam).
(5)
The reproducing apparatus according to any one of (1) to (4), in
which when a reproduction signal is formed from a light receiving
signal output from a light receiving element, an optical path is
divided into a first optical path and second optical path, a high
frequency component is extracted from a light receiving signal on
the first optical path, a low-frequency component is extracted from
a light receiving signal on the second optical path, and
the high frequency component converted into a digital signal and
the low frequency component converted into a digital signal are
synthesized to obtain a reproduction signal.
(6)
The reproducing apparatus according to (5), further including a
reference light beam servo that applies a reference light beam
servo by a light receiving signal on the second optical path.
(7)
The reproducing apparatus according to any one of (1) to (4), in
which when a reproduction signal is formed from a light receiving
signal output from a light receiving element, an optical path is
divided into a first optical path and second optical path, a high
frequency component is extracted from a light receiving signal on
the first optical path, a reference light beam servo is applied by
the light receiving signal of the second optical path, and
the high frequency component converted into a digital signal and a
fixed DC value are synthesized to obtain a reproduction signal.
(8)
The reproducing apparatus according to any one of (1) to (4), in
which when a reproduction signal is formed from a light receiving
signal output from a light receiving element, an optical path is
divided into a first optical path and second optical path, a fixed
DC value is subtracted from a light receiving signal on the first
optical path, a reference light beam servo is applied by the light
receiving signal of the second optical path, and
a signal obtained by converting into a digital signal a signal in
which the fixed DC value is subtracted and the fixed DC value are
synthesized to obtain a reproduction signal.
(9)
The reproducing apparatus according to any one of (1) to (4), in
which when a reproduction signal is formed from a light receiving
signal output from a light receiving element, a reference light
beam servo is applied by a light receiving signal of a single
optical path,
a DC value corresponding to a target phase of the reference light
beam servo is subtracted from the light receiving signal, and
a signal obtained by converting into a digital signal a signal in
which the DC value is subtracted and a signal obtained by
converting the DC value corresponding to the target phase into a
digital signal are synthesized to obtain a reproduction signal.
(10)
The reproducing apparatus according to any one of (1) to (4), in
which when a reproduction signal is formed from a light receiving
element output from a light reception element, a reference light
beam servo is applied by a light receiving signal of a single
optical path,
a fixed DC value is subtracted from the light receiving signal,
and
a signal obtained by converting into a digital signal a signal in
which the DC value is subtracted and a signal obtained by
converting the fixed DC value into a digital signal are synthesized
to obtain a reproduction signal.
(11)
The reproducing apparatus according to any one of (1) to (4), in
which when a reproduction signal is formed from a light receiving
signal output from a light receiving element, a reference light
beam servo is applied by a light receiving signal of a single
optical path, and
a signal obtained by converting the light receiving signal into a
digital signal and a signal obtained by converting a DC value
corresponding to the target phase into a digital signal are
synthesized to obtain a reproduction signal.
(12)
A reproducing method including:
irradiating a recording medium on which signals are each recorded
on a land and a groove with a light beam emitted from a light
source to obtain a signal light beam reflecting each of the
recording signals of the land and the groove, generating a
reference light beam from the light beam emitted from the light
source, and generating, by an optical system, a set of a first
signal light beam and a reference light beam which give a phase
difference of approximately 0.degree. to a superimposed light beam
obtained by superimposing the signal light beam and the reference
light beam, a set of a second signal light beam and a second
reference light beam which give a phase difference of approximately
180.degree. to the superimposed light beam, a set of a third signal
light beam and a third reference light beam which give a phase
difference of approximately 90.degree. to the superimposed light
beam, and a set of a fourth signal light beam and a fourth
reference light beam which give a phase difference of approximately
270.degree. to the superimposed light beam;
using a first light receiving element to receive the set of the
first light beam and the first reference light beam, a second light
receiving element to receive the set of the second signal light
beam and the second reference light beam, a third light receiving
element to receive the set of the third signal light beam and the
third reference light beam, and a fourth light receiving element to
receive the set of the fourth signal light beam and the reference
light beam;
calculating a first difference signal a which is a difference
between a first light receiving signal obtained by the first light
receiving element and a second light receiving signal obtained by
the second light receiving element, and a second difference signal
b which is a difference between a third light receiving signal
obtained by the third light receiving element and a fourth light
receiving signal obtained by the fourth light receiving element,
and
using the first difference signal a, the second difference signal
b, a phase difference .PSI. between a crosstalk component and an
average phase of the signal light beam, and an optical path length
difference .theta. between the signal light beam and the reference
light beam to carry out an arithmetic operation of
asin(.PSI.-.theta.(t))-bcos(.PSI.-0(t))
to obtain a reproduction signal; and
obtaining a successive change amount .DELTA..theta. of .theta. and
updates .theta. with the successive variation .DELTA..theta..
REFERENCE SIGNS LIST
1 Optical recording medium 41 Laser diode 44 Objective lens 71
Phase extraction circuit
* * * * *